The present invention relates to an electric power steering apparatus which uses an electric motor as a source of an steering assisting force, and more particularly concerns an electric power steering apparatus having a construction in which a steering operation in a rack-pinion type steering mechanism is assisted by a rotational force of the electric motor.
An electric power steering apparatus, which drives an electric motor for steering assistance based upon the result of detection of a steering torque applied to a steering wheel and transmits a rotational force of the electric motor to a steering mechanism so as to assist the operation of the steering mechanism, has the advantage that the assisting force characteristics can be controlled more easily depending on travelling conditions, such as the speed of a automobile and the frequency of steering operations, as compared with a hydraulic power steering apparatus using a hydraulic actuator as a source of a steering assisting force.
However, the disadvantage of the above-mentioned electric power steering apparatus is that it is difficult to obtain a small-size electric motor (hereinafter, referred to as motor) which is capable of generating a sufficient rotational force for the steering assistance and that it is difficult to provide a space for the motor on the vicinity of the steering mechanism to which the force is transmitted. Conventionally, in order to solve these problems, a transmission device is interpolated in a halfway of the transmission system between the output side of the motor for steering assistance and the steering mechanism so that the rotational force acquired from the output shaft of the motor is increased by reducing the speed and transmitted to the steering mechanism; thus, the motor is miniaturized while the insufficient output force is supplemented.
With respect to steering apparatuses for automobiles, various types of them have been put into practical use, and those of the rack-pinion type have been known. The steering apparatuses of this type has a construction with which: a rack shaft having rack teeth formed thereon with a predetermined length in the axial direction is installed in a manner so as to extend in the lateral direction with respect to the chassis, both ends of the rack shaft are connected to travelling wheels through tie rods, a pinion shaft which engages the rack teeth is connected to a column shaft connected to a steering wheel so that the rotational movement of the column shaft in accordance with a steering operation is transmitted to the rack shaft through the pinion shaft; thus, the rack shaft is allowed to shift in the axial direction within a range of the formed length of the rack teeth so that the steering operation is carried out.
In the case that such a rack-pinion type steering apparatus is constituted as an electric power steering apparatus, a mechanism which transmits the rotational force of the steering assisting motor to a halfway portion on the column shaft or the pinion shaft that rotates about the shaft is generally adopted. With respect to the transmission device, a worm gear, which has a worm connected to the output end of the motor and a worm wheel fitted to the column shaft or the pinion shaft that are engaged with the worm wheel, and which is capable of providing a high reduction ratio with a simple construction, is widely used.
Here, in the electric power steering apparatus having the above-mentioned construction, the steering control may become inoperative in the event of a locked state of the steering assisting motor; therefore, it is essential to provide a safety measure to avoid the occurrence of such an inoperative state. For this reason, conventionally, an electromagnetic clutch is installed between the transmission device and the output shaft of the motor, and the electromagnetic clutch is disengaged at the time of a locked state of the motor so that the motor in the locked state is cut off from the steering mechanism.
FIG. 1 schematically shows the construction of a transmission system from the motor to the pinion shaft in the electric power steering apparatus having the above-mentioned structure. As illustrated, the electric power steering apparatus is designed so that a worm gear, which is used for increasing the rotational force of a motor M, is provided with a worm W2 that tangentially engages teeth formed on the circumference of a worm wheel W1; therefore, it is essential to maintain a shaft-to-shaft distance L1 corresponding to the sum of radii of the worm wheel W1 and the worm W2 between a pinion shaft 1 (or a column shaft) and the motor M that is coaxially provided with the worm W2, and the motor M needs to be placed at a position far apart from the axial center of the pinion shaft.
In an electric power steering apparatus of the pinion transmission type in which the transmitted rotational force from the motor M is applied to the pinion shaft 1 as shown in the drawing, it is difficult in most cases to install the motor M on the periphery of the pinion shaft 1 that is placed together with the rack shaft in an engine room that has merely limited installation space with the above-mentioned shaft-to-shaft distance L1 therefrom. Moreover, in an electric power steering apparatus of the column transmission type in which the transmitted rotational force from the motor M is applied to the column shaft, the existence of the motor M, placed apart from the column shaft positioned inside the automobile, makes it difficult to provide a sufficient room for driver""s legs.
Furthermore, as illustrated in FIG. 1, the above-mentioned electromagnetic clutch C, which carries out the cut-off operation at the time of the locked state of the motor M, is connected to the output side of the motor M; therefore, the existence of the electromagnetic clutch C increases the length L2 of the motor M in the axial direction, making it difficult to ensure the installation position of the motor M.
As described above, in the conventional electric power steering apparatus, an attempt to miniaturize the steering assisting motor that is achieved by adopting a transmission device is cancelled out by the existence of the transmission device and the electromagnetic clutch, thereby failing to effectively achieve the objective to reduce the space occupied by the motor.
The present invention has been made with the aim of solving the above problems, and it is one object of the present invention to provide an electric power steering apparatus which can miniaturize a steering assisting motor as well as miniaturizing a transmission system to a column shaft or a pinion shaft and which easily ensures a space to be occupied by the motor on the periphery of the column shaft or the pinion shaft.
The electric power steering apparatus of the present invention, which is provided with a column shaft that rotates in response to the rotational operation of a steering wheel, a pinion shaft connected to the column shaft, and a rack shaft that moves in the axial direction within a range of its moving stroke corresponding to a formed length of rack teeth that engages the pinion shaft in response to the rotational movement thereof, and which transmits the rotational force of the steering assisting electric motor to the column shaft or the pinion shaft through a transmission device so as to assist the steering operation carried out by the movement of the rack shaft is characterized in that the transmission device is constituted by hypoid gears consisting of a smaller gear on the electric motor side and a larger gear on the column shaft or pinion shaft side and in that the rack teeth are formed so that the pressure angle thereof reduces toward both ends of the moving stroke.
In the present invention, with respect to the transmission device for reducing the rotational speed of the steering assisting electric motor and for transmitting it to the column shaft or the pinion shaft, the hypoid gears, which consist of a smaller gear on the electric motor side and a larger gear on the column shaft or pinion shaft side so that the shaft-to-shaft distance can be maintained small, is adopted. Moreover, the pressure angle of the rack teeth that are formed on the rack shaft and allowed to engage the pinion shaft is set so as to reduce toward the both ends of the moving stroke of the rack shaft; thus, the displacement of the rack shaft per unit rotation of the pinion shaft is minimized at the both ends of the moving stroke, that is, larger steering ranges in which a lot of force is required for steering, so that the insufficient reduction ratio due to the adoption of the hypoid gears is supplemented and the motor is miniaturized. With these arrangements, the space to be occupied by the motor is reduced on the periphery of the column shaft or the pinion shaft, thereby making it possible to easily ensure the installation space of the motor.
Another electric power steering apparatus of the present invention has a construction in which: the larger gear is externally fitted to the column shaft or the pinion shaft, and a slip ring which is provided in the fitting section and which serves as a torque limiter for allowing the fitting section to slide upon application of a force in a circumferential direction that exceeds a predetermined limit, is provided.
In the above-mentioned invention, the slip ring, which is provided in the fitting section between the column shaft or the pinion shaft and the larger gear of the hypoid gears, allows the larger gear to slide on the circumference of the column shaft or the pinion shaft upon application of an excessive force in the circumferential direction that is exerted at the time that the steering assisting motor is locked or at the time that a reverse input is occurred from the travelling wheel side, achieves the cut-off of the motor in the locked state without using an electromagnetic clutch, and protects the teeth of the hypoid gears used for the transmission device at the time that the reverse input from the wheel side is occurred.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.